This invention relates to a method of concentrating processed vegetable and/or fruit products by reverse osmosis.
Among the means for concentrating squeezed vegetable and/or fruit juice, those by reverse osmosis without involving phase changes are coming to be noted because concentrated products of a higher quality can be obtained by this method than by more commonly practiced methods by heating and evaporation. The present invention relates to improvements in such means for concentration by reverse osmosis.
There are many kinds of conventionally known methods of concentrating vegetable and/or fruit juice by reverse osmosis, including those by causing the squeezed juice to flow through a concentration unit with a plurality of tubular membrane modules connected together by a single-stage single-pass method (as disclosed in Japanese Patent Publications Tokkai 52-136942 and 59-25675), by a multi-stage single-pass method (as disclosed in Japanese Patent Publications Tokkai 5-15351 and 5-336937) and by a multi-stage circulation method (as disclosed in Japanese Patent Publications Tokkai 3-21326 and 3-58774). By the single-stage single-pass method, use is made of only one stage of concentration unit with a plurality of tubular membrane modules connected in series and the squeezed juice is caused to flow through in a single-pass process for reverse osmosis. By the multi-stage single-pass method and the multi-stage circulation method, concentration units each with a plurality of tubular membrane modules connected in series are provided in two or more stages and the squeezed juice is passed through either in a single-pass process or in a circulating process for reverse osmosis.
With any of these prior art means for concentration by reverse osmosis, a high-pressure pump is connected on the inlet side of the concentration unit for supplying the squeezed juice to the tubular modules and a pressure control valve is connected on the exit side of the unit for discharging the concentrated product from the tubular modules, and a plunger pump is usually used as the high-pressure pump. Such prior art means usually have problems of the following four kinds. (1) The use of a plunger pump causes a pulsating motion, thereby disturbing the operating conditions at the time of concentration by reverse osmosis and hence a stable operation cannot be carried out. (2) Because the concentrated product by reverse osmosis under a high-pressure condition (or “pressured condition”) is discharged suddenly through the pressure-control valve into an atmospheric condition, the discharged product tends to get scattered. Air is easily absorbed in this process, requiring a troublesome process management for preventing deterioration of product quality and bacterial contamination. (3) In the field of food processing, there are demands for concentrating processed vegetable and/or fruit products which contain solid components such as seeds, surface skin parts (epidermis) and meat parts (sarcocarp) by reverse osmosis but such solid components get stuck easily at the inlet of the plunger pump and the discharge valve as well as the parts of the pressure-control valve with a small clearance. As a practical matter, therefore, it is very difficult to concentrate by reverse osmosis such a processed vegetable and/or fruit product containing solid components. (4) If an attempt is forcibly made to concentrate such a processed vegetable and/or fruit product by reverse osmosis, its solid components are cut or crushed by the plunger pump or the valve parts of the pressure-control valve and hence concentrated products containing the solid components as they existed before the concentration process cannot be obtained.